1. Field of the Invention
The present invention relates to a boat propulsion system.
2. Description of the Related Art
A boat propulsion system mounted on an outboard motor or the like generates the propulsive force necessary to move a boat. For example, as described in JP-A-Hei 9-301282 and JP-A-Hei 11-129988, in which an engine rotates a drive shaft that extends vertically, a rotational direction conversion mechanism uses a bevel gear to convert rotation of the drive shaft into rotation of a propeller shaft that extends horizontally, and a propeller mounted on the propeller shaft is driven to rotate.
A forwarding bevel gear or a reversing bevel gear meshes together with a dog clutch in the rotational direction conversion mechanism mentioned above. The propeller shaft is thereby provided with rotation of the forward side or rotation of the reverse side. In other words, a forwarding-reversing shift mechanism, which is a portion of the rotational direction conversion mechanism, makes the dog clutch mesh together with either one of the bevel gears to cause the boat to travel forward or to travel backward from a neutral state where the dog clutch does not mesh together with either of the bevel gears.
In the conventional art mentioned above, there is a problem in which a thrust force applied to the propeller shaft varies according to changes in drive force of the engine. When an operation is performed to keep the engine operating at a certain rotational speed, variations in rotation and in torque of the engine are generated, which generates variations in a drive force of the propeller. Thus, variations in thrust force are generated. If variations in thrust force are generated as described above, the bevel gear and the propeller shaft come in contact with each other or separate from each other at a portion where the bevel gear and the propeller shaft meet. It has become known that a user's comfort is reduced by sound and vibration that occur every time the contact is made. Sound and vibration of an engine are large in the case of two-cycle engines. Therefore, sound and vibration caused by variations in thrust force do not draw attention. However, engine sound and vibration are smaller in four-cycle engines. Therefore, sound and vibration caused by variations in thrust force in four-cycle engines draw the attention of the users.